The present invention relates to a device for reversing the direction of flow of a flushing fluid at the level of a drill bit, such as, for example, but not exclusively, a drilling tool creating a vacuum, or negative pressure, at the level of the bit working face during ground drilling operation.
Such a drill tool, described in French Patent Specification 1,567,862, comprises a tool body, at least one ground cutting area provided on said body and delimiting the bit working face, and means for creating in an internal recess of said tool a negative pressure as compared to the pressure prevailing at the level of this working face.
The working face is connected to the zone of negative pressure through at least one duct for lifting the ground cuttings, while a duct for discharging the drilling or flushing fluid passes through the negative pressure zone and opens into the annular space between the drill string and the wall of the drilled borehole through at least one orifice opening outside the tool working face, the flow rate of drilling fluid between the orifice of the discharge duct and the tool working face being limited.
There is thus obtained, under normal operating conditions, a centripetal flushing of the tool working face, i.e., a flow of flushing fluid directed from the borehole wall towards the borehole axis.
It has been however experimentally ascertained that it may sometimes be necessary to temporarily provide a centrifugal flow of flushing fluid, i.e. a flow directed from the borehole axis towards the borehole wall.
This is for example required when drilling through a ground layer containing a fluid such as hydrocarbons at a higher pressure than that prevailing at the level of the bit working face or also when a leakage appears in the fluid flow through the geological formations.
In known devices, such as those described in U.S. Pat. No. 2,805,043 and French Pat. No. 2,071,262, displacement of a movable element uncovers apertures provided in the wall of the drill string, so that the flow of drilling fluid is partly or fully diverted through these apertures. Such devices make it possible to reduce, or even discontinue flushing of the drill bit, but do not permit reversal of the direction of flow of this drilling fluid at the level of the drill bit.
Means for reversing the direction of flow of the drilling fluid are already known. An embodiment of such means is illustrated in FIG. 3 of the Certificate of Addition 2,092,646 to French Patent Specification No. 1,567,862. Such means generally makes use of an obturator such as a ball introduced into the drill string to close a first orifice and thus produce, under the action of the drilling fluid, the displacement of a movable member, which uncovers a second orifice through which the drilling fluid is discharged to flush the drill bit with a reverse direction of flow.
Whenever the initial direction of flow of the drilling fluid must be restored, it is necessary to remove the obturator by means of a suitable fishing tool, or by raising the whole drill column to the surface.
Such operations are in any event time-consuming.
The device of the present invention makes it possible to rapidly reverse the direction of flow of the drilling fluid, so as to shift from a centrifugal irrigation to a centripetal irrigation of the tool and vice-versa, whenever required during the drilling operation.